The objective is to characterize the antigens of the rickettsiae and other bacteria which are pathogenic for humans. Structure and functional aspects of the immunomodulation by the cell wall and soluble components are currently being analyzed as potential candidates for subunit vaccines. Studies employing chemical extracts of Coxiella burnetii have revealed a particulate component which is nontoxic and provides protection against a lethal challenge in mice and guinea pigs. Soluble antigens have been identified as circulating immune complexes during Q fever infections. Ultrastructural analysis of C. burnetii has revealed endospore formation during the developmental cycle of this highly pathogenic bacterium. This discovery will facilitate future studies since we now have some understanding of the historical relationship between C. burnetii resistance to extracellular environments and its highly infectious capability.